BNG in Practice: What the First Year of Mandatory Biodiversity Net Gain Has Shown

Mandatory Biodiversity Net Gain became a legal requirement for major development in England on 12 February 2024 and for minor development on 2 April 2024, under Schedule 7A of the Town and Country Planning Act 1990 (inserted by the Environment Act 2021). Every qualifying planning permission must now deliver a minimum 10% net gain in biodiversity value, measured using the statutory Biodiversity Metric.

A year into mandatory BNG, the practical reality is becoming clear. Some of the early fears have not materialised. Some unexpected challenges have emerged. And the system is still evolving as local authorities, developers, ecologists, and Natural England find their way through the new framework.

The legal requirement is deceptively simple: a minimum 10% net gain in biodiversity value, maintained for at least 30 years. But the detail matters.

Biodiversity value is measured using the **Biodiversity Metric 4.0** (published by Natural England in March 2024, replacing Metric 4.0 from November 2023). The metric calculates a site's biodiversity value in "biodiversity units" based on habitat type, condition, area, and strategic significance. The applicant must demonstrate that the post-development biodiversity value exceeds the pre-development baseline by at least 10%.

The gain can be delivered through three mechanisms, applied in a strict hierarchy: **on-site habitat creation or enhancement** (preferred), **off-site habitat creation through registered biodiversity gain sites**, or **purchase of statutory biodiversity credits** from Natural England (last resort). The hierarchy is important — applicants must demonstrate that on-site delivery has been maximised before resorting to off-site solutions.

NPPF paragraph 186 reinforces the framework by stating that plans should promote the conservation, restoration and enhancement of priority habitats, ecological networks and the protection and recovery of priority species. BNG sits within this broader ecological policy context.

The Biodiversity Metric 4.0 is a detailed calculation tool, and getting it right requires genuine ecological expertise. Our analysis of BNG submissions shows that the most common errors are in baseline habitat classification and condition assessment. Classifying a habitat incorrectly — recording modified grassland as "other neutral grassland" when it should be "modified grassland," for example — can significantly change the number of units and the scale of the gain required.

Condition assessment is particularly subjective. The metric provides condition assessment criteria for each habitat type, but applying them consistently requires professional ecological judgement. Two ecologists can assess the same grassland and reach different condition scores, which directly affects the baseline biodiversity value.

The early concern that BNG would sterilise large areas of development land has generally not materialised. For the majority of residential schemes, 10% net gain can be achieved through a combination of habitat creation in public open space, enhanced boundary planting, species-rich grassland in SuDS features, and improved management of retained habitats.

The key is **early integration into the design process**. Schemes that bolt BNG on at the end — after the layout is fixed and the landscaping is designed — struggle to achieve on-site delivery. Schemes that consider biodiversity from masterplan stage find that the habitat creation often aligns with other requirements: green infrastructure, SuDS, amenity space, and landscape buffers.

For schemes that cannot achieve 10% on-site, the off-site market is still maturing. Registered biodiversity gain sites are being established, but availability is patchy — particularly for specific habitat types in specific locations. The strategic significance multiplier in the metric means that off-site gains in the right location deliver more units per hectare than those in less strategically important areas.

Statutory biodiversity credits — available from Natural England as the last resort — are deliberately expensive. They are priced to incentivise on-site and off-site delivery rather than credit purchase. At current prices, credits for most habitat types cost significantly more per unit than bespoke off-site solutions.

The biggest practical challenge in the first year has not been ecological — it has been administrative. Many local planning authorities are still building capacity to validate and assess BNG submissions. Ecologist resource within planning departments is limited, and the technical complexity of metric calculations means that non-specialist case officers struggle to assess whether a submission is adequate.

This has led to delays in validation, extended determination periods, and inconsistent application of the requirements between authorities. Some authorities have published detailed BNG validation checklists and guidance; others are still working out their approach.

The Chartered Institute of Ecology and Environmental Management (CIEEM) has published guidance on the role of ecologists in BNG, emphasising that metric calculations should be carried out by competent ecologists with relevant field experience. CIEEM's guidance on preliminary ecological appraisals (2017, updated 2024) and ecological impact assessment (2018, updated 2024) provides the professional framework for BNG work.

In practice, this means that a BNG metric calculation submitted by someone without ecological competence is at risk of being challenged by the local authority's ecological adviser or by Natural England. The metric is a tool — it requires professional judgement to apply correctly.

Not all development requires BNG. Key exemptions include:

**Permitted development** that does not require planning permission is exempt. **Householder applications** (extensions, outbuildings) are exempt. **Self-build and custom housebuilding** on single plots is exempt. Development impacting habitat of an area **below the de minimis threshold** (currently 25 square metres or 5 metres of linear habitat) is exempt.

Small sites (1-9 dwellings or sites below 0.5 hectares for non-residential development) can use the **simplified Small Sites Metric** rather than the full Biodiversity Metric 4.0, which reduces the assessment burden.

Based on the first year of mandatory BNG, several practical recommendations emerge:

**Commission a Preliminary Ecological Appraisal early.** Understanding your baseline habitat before you design the scheme is essential. A PEA will identify the habitats present, their approximate condition, and the likely scale of BNG required.

**Integrate BNG into the masterplan.** Do not treat biodiversity as an afterthought. Work with your ecologist and landscape architect from the outset to identify where habitat creation can be delivered within the scheme layout.

**Allow for 30-year management.** BNG habitats must be managed for at least 30 years under a legally binding Biodiversity Gain Plan. Factor the management costs into your viability assessment and ensure the management mechanism (management company, local authority adoption, or conservation body) is agreed early.

**Check local authority requirements.** Each authority is developing its own BNG validation requirements and supplementary guidance. Check what is required before submitting your application.

Mandatory BNG is a significant change to the planning system, but it is workable. The first year has shown that with early ecological input, integrated design, and professional metric calculations, most developments can achieve 10% net gain without fundamentally compromising scheme viability.